When a force moves an object, energy is transferred. The energy transferred is called the work done, and it is measured in joules (J). The work done is the force multiplied by the distance moved along the line of action of the force:
work done = force × distance, or W = F × s
Force is in newtons (N) and distance is in metres (m). One joule is the work done when a force of one newton moves an object one metre, so 1 J = 1 N m. Only the distance moved in the direction of the force counts. Rearranged, F = W ÷ s.
Example: a force of 30 N pulls a sledge 6 m, so the work done is 30 × 6 = 180 J. Lifting a 20 N book 2 m needs 20 × 2 = 40 J of work.
The energy transfer depends on the situation. Lifting a weight transfers energy to the gravitational potential store. Pushing a box along a rough floor does work against friction, and the energy is transferred to the thermal store, so the box and floor warm up slightly.